Method for the regeneration of humidity-laden process air and arrangement for carrying out said method

ABSTRACT

A method for the regeneration of humidity-laden drying cartridges includes heating to 220 to 300° C. and introducing it into a drying cartridge for regeneration. Subsequent cooling of the drying cartridge is achieved by a partial stream of air diverted from the dried process air. An arrangement suitable for carrying out the method is also disclosed.

BACKGROUND OF THE INVENTION

The invention relates to a method for regenerating moisture-ladenprocess air as well as to an arrangement for carrying out the method.

In numerous manufacturing processes, especially when processingplastics, the starting materials and/or the intermediate products mustbe dried before they are processed further. At the same time, theprocess air, used for the drying process, accumulates moisture, whichwas withdrawn from the starting materials and/or the intermediateproducts. Basically, the so-resulting moisture-laden warm process aircould be discharged untreated to the environment and replaced by freshair. However, such a solution is incompatible with conserving energy andtherefore also excluded for reasons of costs.

When plastic granulates are processed, it is necessary, as is alreadymentioned above, to dry the plastic granulates before they are processedor processed further. This is done by means of warm, dry air (processair), which flows through the plastic granulates in a container,provided for this purpose, takes up the moisture to begin with and thensubsequently must be freed once again from the moisture taken up.

For this purpose, the process air, in which the moisture hasaccumulated, is subjected to a regeneration process for removing thismoisture from the drying cartridges filled with molecular sieves, bywhich the moisture of the process air, which is carried along, isabsorbed.

Basically, several such drying cartridges are used, which are connectedin parallel. While at least one of the drying cartridges is operating inthe drying phase, the absorbed moisture is removed at the same time fromone or more drying cartridges, which are connected in parallel, so thatthese cartridges are prepared for a new drying phase.

It is, for example, known from the state of the art that theregeneration of the drying cartridges may be conducted according to theso-called co-current principle, according to which the direction of flowin the drying cartridges is the same in the drying phase for the processair and the regeneration phase of the drying cartridge.

In the case of the so-called counter-current principle, the direction offlow during the regeneration of the drying cartridges is opposite tothat during the drying process.

The advantage of the counter-current principle lies in the distinctlylower energy consumption during the regeneration. While the dryingcartridge is absorbing moisture from the process air during the dryingprocess, it is moistened continuously in the direction of flow. A front,separating the moist region from the dry cartridge, migrates in thedirection of flow. In good time, before this “front”, so formed andmigrating through the drying cartridge, has reached the end of thedrying cartridge, the flow of the process air must be switched over to adifferent drying cartridge while a safety or buffer zone is maintained.By these means, it is ensured that a dry zone is available withouttemporal interruption and that the drying process can, accordingly, takeplace continuously without interruption.

For the countercurrent regeneration, the still-remaining dry region ofthe drying cartridge is now used as the starting point, from which theadjoining, moistened regions of the drying cartridge are dried onceagain continuously in a direction opposite to the preceding moisteningof the drying cartridge. The front, separating the moist from the dryregion, now migrates back opposite to its original direction ofmovement.

In the case of the previously described co-current regeneration, thefront, separating the moist from the dry region, rolls over the regionof the drying cartridge, which initially remains dry, and initiallystill absorbs moisture from the region of the drying cartridge,preceding the front separating the moist from a dry region.

Finally, the “by-pass method” is also used in some cases, for which theregeneration of the drying cartridge takes place at all times with apartial amount of the process air. For this purpose, 15 to 20% of theprocess air is diverted from the main stream, heated to about 220° to300° C. and supplied to the regenerating drying cartridge. At the end ofthe drying process, the heating, assigned to the drying cartridge, whichis to be regenerated, is switched off and the regenerated dryingcartridge is cooled to about 60° C. with the diverted stream of processair. Only when the temperature has been lowered to such a level, is themolecular sieve in the drying cartridge, which is to be regenerated,once again fully effective. At the same time, however, a constantlyvented partial amount of the process air must be taken up in the cycleas fresh air, so that the process air, to a certain extent, isadditionally loaded with moisture and the drying cartridge in the dryingprocess is saturated more quickly with moisture.

As is evident from the state of the art, there have been numerousefforts to improve the functionality and efficiency of the regeneration.

SUMMARY OF THE INVENTION

It is an object of the invention to create a regeneration process fordrying cartridges, which, with the least possible expenditure forequipment and a relatively low consumption of energy, makes possible anoptimized regeneration of the drying cartridges.

The invention is explained in greater detail with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagrammatic representation of a device for dryingplastic granulate and for regenerating the process air, moistened duringthe drying process of the plastic granulate with the help of a molecularsieve,

FIG. 2 shows an adsorption isotherm of the molecular sieve, and

FIG. 3 shows a graphic representation of the variation in temperatureduring the regeneration.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, the region 10, which is at the right and has a border, showsa drying container 12, to the lower region of which process air, comingfrom a heater 11, is supplied. The process air flows through thegranulate, which is to be dried, and emerges once again from the upperregion of the drying container 12. The process air passes through afilter 13 and then reaches a blower 14, from which the process air ispassed on to the drying region 20.

At least two drying cartridges 24 a and 24 b, to which the process air,which is to be dried, can be supplied over the valves 22 a and 22 b, aredisposed in the drying region 20 for the process air.

In a first phase, the process air, which is to be dried or dehumidified,is supplied over a valve 22 b to a drying cartridge 24 b. As the processair flows through the molecular sieve, disposed in the drying cartridge24 b, the moisture, contained in the process air, is absorbed in themolecular sieve. In the molecular sieve, a moistened inlet region and anadjoining dry region are formed.

At the same time, the front, separating the moist from the dry region,migrates continuously from the inlet region to the outlet region of thedrying cartridge 24 b.

During the drying process in the drying cartridge 24 a, which takesplace at the same time, hot atmospheric air is supplied in a first stepof the process by a blower 21 over a valve 23 a and a heater 25 a. Thefresh air, heated with the heater 25 a to a temperature of about 220° to300° C., is supplied to the regenerating drying cartridge 24 acountercurrent to the direction of flow of the process air, until thefront, which separates the dry from the moist region and moves from thebottom to the top, has reached the upper region of the drying cartridge24 a. When the front, separating the dry from the moist region, hasreached the upper region of the drying cartridge 24 a, the second stepof the process is initiated. The heater 25 a is switched off and thevalve 23 a is redirected in such a manner, that dry process air at atemperature of about 60° C. is passed into the drying cartridge 24 a,which is to be regenerated, for the purpose of follow-up drying andcooling. Moreover, this dry process air is introduced in a directionopposite to that established for the process air, which is to be dried.

For the processes, described above, it is important that, in good time,before the front separating the moist from the dry region of the dryingcartridge 24 b, employed for the drying process of the process air, hasreached the outlet region of the cartridge, the valves 22 a, 22 b, 23 aand 23 b are controlled, so that the process air, which is to be dried,is no longer supplied to the drying cartridge 24 b and, instead, issupplied to the previously regenerated, other drying cartridge 24 a,through which it now flows from top to bottom.

The processes, described above, proceed now with roles exchanged betweendrying cartridges 24 b and 24 a and related heaters 25 b and 25 a.

Instead of the drying cartridges 24 a and 24 b, given in the example,further, additional drying cartridges can also be provided. Byappropriately selecting the number of drying cartridges for the dryingand the regenerating cycle, it is possible to take into account thedifferent periods, during which the drying cartridges 24 a and 24 b areused for drying the process air on the one hand and regenerating thedrying cartridges 24 a and 24 b on the other.

Since the regenerating process of the drying cartridges 24 a and 24 btakes place relatively rapidly in relation to the drying process, thecooling phase of the regenerated drying cartridges, which follows theregeneration, can already be used for a drying function of theseregenerated drying cartridges, which are to be cooled, and, moreover,optionally in parallel to the other drying cartridge or cartridges.

After the drying cartridge 24 a or 24 b has been dried with hot air at220° to 300° C., it must be cooled to a temperature of about 60° C.,since the molecular sieve of the drying cartridges 24 a, 24 b regainsits full effectiveness once again only at such a lowered temperature.

A heat exchanger may be provided for the re-cooling process. The unusedthermal energy from the regeneration of the drying cartridges 24 a, 24 bcan be transferred with such a heat exchanger to the stream of airdrying the material to be dried. However, this can lead to problemsparticularly in the case of material dried at low temperatures. It maybe necessary to decrease the capacity of the heat exchanger or even toprovide a refrigerator.

When atmospheric air is used partly or exclusively, the quality of theregeneration, especially during the cooling phase, also depends on thehumidity of the atmospheric air. This becomes clear from the adsorptionisotherms shown in FIG. 2.

The process air is heated to about 220° to 300° C. and preferablyhowever 250° C., before it is passed through the drying cartridge 24 aor 24 b for the purpose of regenerating the cartridge. By these means,the forces binding the water to the molecular sieve in the dryingcartridge 24 a, 24 b are canceled. The water can be taken up by theregenerating air. Energy is consumed. In FIG. 2, the molecular sieve isin state A1. The time, at which the drying cartridge 24 a, 24 b as awhole no longer gives off any water, can be determined from thevariation in temperature at the discharge side of the drying cartridge.At this time, the temperature rises clearly more rapidly, as can be seenin FIG. 3.

In the case of the known method, the molecular sieve reaches the valueA2 in the representation of FIG. 2. However, this state also depends onexternal climatic conditions. Since only state A2 is reached by methodsworking exclusively with atmospheric air, the dew point of the drier isclearly higher than in the case of the inventive method, for which stateB2 is reached. The cause of this is the residual moisture in themolecular sieve.

A partial stream of air, which is diverted from the process air, is usedto cool the drying cartridge 24 a, 24 b. With that, the result of theregeneration depends only on the dew point of the process air. Since aconstant dew point of the process air is aimed for and largely achieved,the result of the regeneration is also practically constant.

The energy used for the regeneration is optimized by selecting thecountercurrent principle, which has already been explained above.

For the regeneration, atmospheric air is aspirated with the regenerationblower 21, supplied to the regeneration heater 25 a by suitable settingsof the valve 23 a and heated there to a temperature of about 220° to300° C. The air, so heated, reaches the drying cartridge 24 a, which isto be regenerated, where it absorbs moisture from the drying cartridge24 a. At the same time, use is made of the fact that the molecular sievecan absorb less water at higher temperatures. The air, laden with water,is discharged to the atmosphere by a suitable setting of valve of 22 a.

In the next step of the process, the regeneration heater 25 a is thenswitched off and the flow of air controlled in such a manner, thatalready dried process air, having a temperature of about 50 to 60° C.,is used for the recooling of the drying cartridge 24 a.

Subsequent drying of the drying cartridge 24 a takes place inconjunction with the heat stored in the molecular sieve. The subsequentdrying is independent of external conditions of temperature andhumidity. The molecular sieve strives to reach a lower level withrespect to the water content. In FIG. 2, the molecular sieve is now instate B1 and, when all the heat is used up, in state B2.

The flow of air can be controlled with valves 22 a, 22 b, 23 a, 23 b insuch a manner, that the regenerated drying cartridge 24 a can be usedonce again for drying the process air. Alternately, the process with thedrying cartridge 24 b proceeds in a very similar manner.

1. A method of regenerating a first drying cartridge, said first dryingcartridge being previously moistened by providing a first flow ofprocess air in a first flow direction for generating a stream of driedprocess air in said first flow direction, said regenerating methodcomprising: heating atmospheric air from approximately 220° toapproximately 300° C. for providing heated air; providing said heatedair to said first drying cartridge in a flow direction that iscountercurrent to said first flow direction; drying a second flow ofprocess air bypassing said second flow of air through a second dryingcartridge; and diverting a partial stream of said second flow of driedprocess air to said first drying cartridge, subsequent to said step ofsupplying said heated air to said first drying cartridge, for recodingsaid first drying cartridge.
 2. The method of claim 1, wherein theatmospheric air is heated to 260° C.
 3. The method of one of claims 1 or2, wherein the recooling is carried out wit a partial stream of airhaving a temperature of about 50° to 60° C.
 4. The method of one ofclaims 1 or 2, wherein after a relatively short regeneration time, theregenerated drying cartridge can already be used in a cooling phase onceagain for drying process air.
 5. Arrangement for carrying out the methodof one of claims 1 or 2, wherein said first and second drying cartridgesare present, which are provided, on the one hand, with first three-wayvalves, between which a cross-connection is disposed and, on the other,with second three-way valves, between which a cross-connection isdisposed, the second three-way valves being connected indirectly over aheater with said drying cartridge.
 6. The arrangement of claim 5,wherein a blower introduces said atmospheric air into saidcross-connection between the second three-way valves.
 7. The method ofclaim 1, wherein said second flow of dried process air is provided tosaid first drying cartridge in a flow direction that is countercurrentto said first flaw direction.